The letter-spacing property increases or decreases the spacing
(width) of the blank (white) space between adjacent characters or letters.
A positive value increases the spacing. A negative value decreases the
spacing.

normal

The normal value is the default. With one exception, by declaring
normal, you can ensure that any previous declaration will not
effect the spacing. Even if normal is declared, the spacing could
still be effected by justification by the text-align property.
See the length value, below, for a way of turning off justification.

length

The length value can be negative or positive. A value of zero
prevents justification of the text from occurring, even if justification
has been declared. The length can be in any of the following
seven units. Use the abbreviation.